The invention is in the field of ground conductor monitoring systems and is particularly directed to a system of this type making efficient use of an audio signal to monitor the quality of at least the ground conductor of a cable system. The invention is particularly advantageous when used in a cable system for supplying power to mining equipment, but is also useful in other systems where it is desirable to monitor the quality of at least the ground conductor of a cable system, typically for the purpose of automatically cutting off power in case of a failure of the ground conductor.
There have been various proposals in the past for monitoring the electrical continuity of a ground conductor or a similar conductor so as to cut off power in case of failure. Some examples of such proposals are discussed in U.S. Pat. Nos. 2,880,377; 3,387,064; 3,746,929; 3,783,340 and 4,075,675. The invention described in greater detail below is believed to be an improvement over such proposals in that it makes more efficient use of the monitoring signal and is believed to provide a greater measure of safety.
In one specific embodiment of the invention a closed loop for a monitoring signal is provided by connecting the load ends of the power conductors of a cable to the load end of the ground conductor through a filter which blocks power but passes the monitoring signal, and connecting the source ends of the power conductors to the source end of the ground conductor through a series circuit of a similar filter and a pick-up coil wound on a transmitter toroid which surrounds the ground conductor. A transmitter coil, also wound on the transmitter toroid, is driven by a suitable transmitter to induce a monitoring signal in the pick-up coil and thus in the closed loop. A monitor toroid also surrounds the ground conductor and has a monitor coil which generates a tripping signal in response to an interruption in the flow of the monitoring signal in the ground conductor. The tripping signal may go to a circuit breaker actuator which cuts off power to the power conductors. The filter at the load end of the cable may be a tuned filter having a series resonance matched to the monitoring signal such that a fault in less than all of the power conductors will raise the impedance the tuned filter presents to the monitoring signal, and the resulting weakening of the monitoring signal will be detected as a ground fault by the monitor.